The present invention relates to improved glue preparing equipment and, more particularly to a main tank suitable for use in equipment for manufacturing starchy glue as an adhesive for corrugated cardboard.
As is commonly known, two-sided corrugated cardboard is manufactured by spreading glue over the surfaces of corrugations of a corrugated core sheet of paper, and putting the core sheet between two liner sheets of paper which are glued to the core for forming the product. This glue is manufactured from corn starch or potato starch, for example. Typical manufacturing methods include: (1) the Stein-Hall method, (2) the no-carrier method, and (3) the Mino-Carr method.
Three conventional glue preparing procedures are described as follows. Method (1) comprises, for example, the steps of filling a main tank with 1,000 liters of 50.degree. C. hot water, charging 100 kg of starch while stirring the mixture, then adding 45 kg of caustic soda of a concentration of 33.3%, agitating the resultant mixture for about 20 minutes until the formation of glue, which is known as carrier. After diluting the carrier with 1,300 liters of water, 500 kg of main starch are added, and borax is added after the lapse of a few minutes. Agitation of the mixture for another 20 minutes completes glue preparation.
Method (2) comprises, for example, the steps of charging 600 kg of starch into 1,900 liters of 35.degree. C. hot water, agitating the resultant mixture, then adding 454 kg of 4.2% caustic soda, adding borax after agitation for 20 minutes, and further agitating for another 20 minutes to complete glue preparation.
Method (3), which comes between methods (1) and (2), comprises, for example, the steps of first charging 300 kg of starch into 2,300 liter of 35.degree. C. hot water, adding 54 kg of 31.5% caustic soda, stirring the mixture for 20 minutes, then adding 300 kg of secondary starch, agitating the mixture for 20 minutes, and adding borax while agitating, to complete glue preparation.
Glue preparation based on these conventional methods is basically accomplished in a main tank as shown in FIG. 15. The main tank is composed of an agitator comprising a cylindrical tank body 17, agitating blades 18 and 19, a shaft 20 and a motor 21. Water (hot water) W, starch D, caustic soda N, and boric acid B are charged from above into the tank 17. Glue is prepared by methods (1) to (3) as described above using this type of conventional tank. When it is necessary to heat the mixture during the steps of glue preparation, steam S is injected through the wall of the tank 17. Upon completion of glue preparation, the liquid is transferred through a valve 22 by a pump 23 into a storage tank T.
The agitating blades 18 and 19 are rotated at a relatively high speed. The agitating blade 18 is a disc having saw teeth and has the function of agitating the starch solution as well as imparting a shear to accelerate dispersion of the starch and control the viscosity thereof. The agitating blade 19 has the function of only agitating the mixture, and may be omitted.
The procedures shown in FIGS. 16 and 17 are used for method (1) above. The procedure of FIG. 16 comprises the steps of, after charging main starch into the tank 17, continuously extracting the starchy solution from the bottom of the tank 17 through a valve 24, sending the extracted solution to a high-speed shear comprising a tank 25, an agitating blade 26 and a motor 27 by means of a pump 23, and circulating the solution further through a valve 28 to the tank 17. This high-speed shear is more intense than that of the agitating blade 18.
The procedure shown in FIG. 17 comprises the steps of, after preparing carrier in a primary tank which is composed of a tank 29, an agitating blade 30 and a motor 31, feeding the prepared carrier into a tank 33, called the secondary tank and previously filled with diluting water, and charging secondary starch into the mixture while agitating the same with an agitator, which is composed of an agitating blade 34, a shaft 35 and a motor 36.
Then, in the procedure shown in FIG. 18, a high-speed shear which is composed of a blade 38, a shaft 39 and a motor 40, is additionally provided in the secondary tank shown in FIG. 17. This procedure is used for method (2) above, and comprises the steps of charging hot water and starch into a tank 33, adding caustic soda while agitating the mixture with a low-speed agitator which is composed of an agitating blade 34, a shaft 35, and motor 36, and preparing glue by further operating the high-speed shear described above. In FIG. 18, the abbreviation Dr represents a path to the drain pit.
Glue for corrugated cardboard is basically prepared by adding caustic soda while agitating starch dispersed in hot water (hereinafter referred to as "starchy slurry"), but the properties of the glue including adhesiveness and viscosity stability are not satisfactory unless a sufficient shear is imparted. It is necessary to rotate the agitator at a high speed for this purpose. However, because each agitator is suspended from above as shown in FIGS. 15 to 17, high-speed rotation requires using an agitating blade of small diameter due to limitations in vibration and strength. This reduces the pumping volume of the blade, thus resulting in a reduction of the amount of circulated starchy slurry, and also a reduction of the number of saw teeth of the agitating blade shown in FIGS. 15 and 16.
The resultant drawbacks include failure to disperse the starch and to effectively impart shear to the starchy slurry. It is well known that the installation of baffles on the tank inner wall permits more effective agitation. In the agitators shown in FIGS. 15 and 16, however, the low agitating force leads to inconvenient adhesion of undispersed starch onto the baffles.
Particularly, method (1), in which the carrier starchy slurry has a high consistency even after dilution, has the inconvenience that part of the main starch remains undispersed. To avoid this, milling and shearing are conducted with a high-speed shear shown in FIG. 16, or carrier is prepared in a small-volume primary tank as shown in FIG. 17, and the glue is prepared by increasing the degree of dilution in a secondary tank. In both cases, two tanks are required. The method shown in FIG. 16 has a drawback in that the transfer of liquid to a storage tank after completion of glue preparation causes part of the glue to remain in the piping 37, and this residue is mixed with starchy slurry in the next run, thus destabilizing the quality of glue.
When adding caustic soda to starchy slurry and continuing agitation, starch particles swell and are partially converted into glue under the effect of shear. This reaction depends upon the revolutions of the agitator of the main tank. In the conventional method, in which the agitator operates at a constant speed, it is impossible to adjust the glue properties unless the raw material conditions are changed.
Furthermore, since the blade structure of the agitator is fixed, it is impossible to adjust the glue properties by altering the set conditions of the blade, and in addition, the pumping volume is small. This means that the suction volume of the agitator is small. This results in defects such as poor suction of light undispersed matters accumulating at the tank center and low dispersibility of the starchy slurry. The practices shown in FIGS. 16 to 18 using two agitators have the problem of increased mechanical loss of the drive section. In the conventional methods, as described above, shearing accomplished when the slurry is in a free state causes low efficiency, and the small number of teeth and the small tooth length require high-speed rotation, resulting in problems such as large power consumption.
An object of the present invention is to solve the above-mentioned problems in the conventional methods, and to provide a main tank for use with glue preparing equipment which delivers remarkable agitating and shearing effects and can cope with higher speeds by providing a larger-diameter agitating blade and baffles.